NAME

error::sdt - <sys/sdt.h> marker failures

DESCRIPTION

Systemtap's <sys/sdt.h> probes are modeled after the dtrace USDT API, but are implemented
differently. They leave a only a NOP instruction in the userspace program's text segment,
and add an ELF note to the binary with metadata. This metadata describes the marker's
name and parameters. This encoding is designed to be parseable by multiple tools (not
just systemtap: GDB, the GNU Debugger, also contains support). These allow the tools to
find parameters and their types, wherever they happen to reside, even without DWARF
debuginfo.
The reason finding parameters is tricky is because the STAP_PROBE / DTRACE_PROBE markers
store an assembly language expression for each operand, as a result of use of gcc inline-
assembly directives. The compiler is given a broad gcc operand constraint string ("nor")
for the operands, which usually works well. Usually, it does not force the compiler to
load the parameters into or out of registers, which would slow down an instrumented
program. However, some instrumentation sites with some parameters do not work well with
the default "nor" constraint.
unresolveable at run-time
GCC may emit strings that an assembler could resolve (from the context of compiling
the original program), but a run-time tool cannot. For example, the operand string
might refer to a label of a local symbol that is not emitted into the ELF object
file at all, which leaves no trace for the run-time. Reference to such parameters
from within systemtap can result in "SDT asm not understood" errors.
too complicated expression
GCC might synthesize very complicated assembly addressing modes from complex C data
types / pointer expressions. systemtap or gdb may not be able to parse some valid
but complicated expressions. Reference to such parameters from within systemtap
can result in "SDT asm not understood" errors.
overly restrictive constraint
GCC might not be able to even compile the original program with the default "nor"
constraint due to shortage of registers or other reasons. A compile-time gcc error
such as "asm operand has impossible constraints" may result.
There are two general workarounds to this family of problems.
change the constraints
While compiling the original instrumented program, set the STAP_SDT_ARG_CONSTRAINT
macro to different constraint strings. See the GCC manual about various options.
For example, on many machine architectures, "r" forces operands into registers, and
"g" leaves operands essentially unconstrained.
revert to debuginfo
As long as the instrumented program compiles, it may be fine simply to keep using
<sys/sdt.h> but eschew extraction of a few individual parameters. In the worst
case, disable <sys/sdt.h> macros entirely to eschew the compiled-in
instrumentation. If DWARF debuginfo was generated and preserved, a systemtap
script could refer to the underlying source context variables instead of the
positional STAP_PROBE parameters.